introduction to composite_materials in aerospace_applications
TRANSCRIPT
CONTENT
Page no: Introduction 1
Chapter No:1. Composite Material 2-5
2. Fiber Material 6-9
3. Matrix and Filler Materials 10-11
4. The reasons why composites are selected for Aerospace
Applications 12-17
5. Advantage& Disadvantages of Composite 18-19
Conclusion 20
Introduction
Composite material is a material composed of two or more distinct phases(matrix phase and
dispersed phase) which are chemically & physically different.
Composites are used because overall properties of the composites are superior to those of the
individual components.
The primary phase, having a continuous character , is called matrix. Matrix is usually more
ductile and less hard phase. It holds the dispersed phase and shares a load with it.
The second phase (or phases) which is embedded in the matrix is called dispersed phase.
Dispersed phase.
CHAPTER: 1
Composite Material:
Composites, which consist of two or more separate materials combined in a macroscopic
structural unit, are made from various combinations of the other three materials.
The relative importance of the four basic materials in a historical context has been presented by
Ashby (Technology of the 1990s: Advanced Materials and Predictive Design, M.F. Ashby,
Philosophical Transactions of the Royal Society of London, A322, 393-407, 1987) and is shown
schematically below (figure taken from Gibson).
Mankind has used composites since early time; for example, straw-reinforced clay bricks used by
Israelites (the book of Exodus in the Old Testament), plant fiber-reinforced pottery, etc. They
knew from daily use that fiber reinforcement of a material is very effective because many
materials (but not all) are much stronger and stiffer in fiber form than they are in bulk form.
For example, Griffith found that as glass rods and fibers got thinner, they got stronger. He found
that that for very small diameters the fiber strength approached the theoretical cohesive strength
between adjacent layers of atoms, whereas for large diameters the fiber strength dropped to near
the strength of bulk glass.
Fibers allow one to obtain the maximum tensile strength and stiffness of a material, but there are
disadvantages. Fibers alone cannot support longitudinal compressive loads and their transverse
mechanical properties are generally not as good as the corresponding longitudinal (fiber
direction) properties. Thus, there is often the need to place fibers in different directions
depending upon the particular loading application.
Types of Fiber-Reinforced Composites
One generally finds four types of fiber-reinforced composites as shown below (from Gibson).
They differ in how the fibers are utilized to make the composite (orientation and length of
fibers).
Continuous fiber composites are generally "laid-up" in plies (or laminae) with each ply having
fibers oriented in the same direction. A layer of fibers all oriented in the same direction is
imbedded in a homogeneous material (called the matrix) to make a single ply or laminae. For
example, glass-epoxy has a layer of glass fibers running more-or-less parallel within an epoxy
resin matrix material.
Individual plies can be stacked or layered and bonded together with individual ply fiber
directions being selected so as to tailor the lay-up (or laminate) to have desired overall structural
characteristics of the laminate. Under loading, the potential for delamination (or separation of
the laminae) is a major problem because the interlaminar strength is matrix dominated (i.e., if the
matrix is weak, ply delamination can occur).
Woven fiber composites are similar to ordinary cloth used in the textile industry. The woven
fiber may be 2-D (fibers interwoven in 2 directions) or 3-D (fibers interwoven in 3 directions).
Woven fiber composites do not generally have distinct laminae and are not nearly as susceptible
to delamination; however, strength and stiffness are sacrificed due to the fact that the fibers are
not as straight (because of the weaving) as in the continuous fiber laminate.
Chopped fiber composites have fibers that are relatively short and have a random orientation
and distribution of the fibers. Chopped fiber composites generally have mechanical properties
that are considerable poorer than those of continuous fiber composites. However they are
cheaper to manufacture and are used in high-volume applications.
Hybrid composites generally consist of mixed chopped and continuous fibers; or mixed fiber
types such as glass/graphite.
Sandwich composites are also common. They consist of high strength composite facing sheets
(which may be any of the four fiber composites discussed above) bonded to a lightweight foam
or honeycomb core (from Gibson).
Sandwich structures have extremely high flexural stiffness-to-weight ratios and are widely used
in aerospace structures. The design flexibility offered by these and other composite
configurations is obviously quite attractive to designers, and the potential now exist to design not
only the structure, but also the structural material itself.
Almost all of the fiber-reinforced composite types discussed above can be utilized in complex
curved geometries although the manufacturing process may be much more costly and difficult.
For example, wound fiber-reinforced pressure vessels are common and are manufactured by
winding either individual fiber filaments on a mandrel (having the shape of the vessel) or
individual plies are wound on the mandrel. Curved composite material panels on aircraft wings,
fuselage and nacelles are common.All of the composite types have various manufacturing
processes required to bond individual plies.
CHAPTER:2
Fiber Material :
Glass fibers consist primarily of silica (silicon dioxide) and metallic-oxide-modifying elements
are generally produced by mechanical drawing of molten glass through a small orifice. E-glass
accounts for most of the glass fiber production and is the most widely used reinforcement for
composites. The second most popular glass fiber, S-glass, has roughly 30 percent greater tensile
strength and 20 percent greater modulus of elasticity than E-glass but is not as widely used
because of its higher cost.
Graphite or carbon fibers are the most widely used advanced fiber, and graphite/epoxy or
carbon/epoxy composites are now used routinely in aerospace structures. The actual fibers are
usually produced by subjecting organic precursor fibers such as polyacrylonitrile (PAN) or rayon
to a sequence of heat treatments, so that the precursor is converted to carbon by pyrolysis.
Graphite fibers are typically subjected to higher heat treatments than are carbon fibers. Carbon
fibers are typically 90-95% carbon, whereas graphite fibers are at least 99% carbon.
Aramid polymer fibers, produced primarily by E.I. duPont deNemours & Company under the
tradename "Kevlar," were originally developed for use in radial tires. The density of Kevlar is
about half that of glass and its specific strength is among the highest of currently available fibers.
Kevlar also has excellent toughness, ductility, and impact resistance; unlike brittle glass or
graphite fibers.
Boron fibers are actually composites consisting of a boron coating on a substrate of tungsten or
carbon. The diameter of boron fibers is among the largest of all the advanced fibers, typically
0.002-0.008 in. Boron fibers have much higher strength and stiffness than graphite, but they also
have higher density. Boron/epoxy and boron/aluminum composites are widely used in aerospace
structures, but high cost prevents more widespread use.
Silicon carbide (SiC) fibers are used primarily in high-temperature metal and ceramic matrix
composites because of their excellent oxidation resistance and high-temperature strength
retention. SiC whisker-reinforced metals are increasingly being used as alternative to un-
reinforced metals and continuous fiber-reinforced metals. SiC whiskers are quite small, typically
8-20 in. diameter and about 0.0012 in. long so that standard metal-forming processes such as
extrusion, rolling and forging can be easily used.
The list of fibers goes on … On the following pages are a) Selected properties of fibers and
bulk metals, b) Specific strength vs. specific modulus for various fibers and c) Specific strength
vs. specific modulus (stiffness) for various composites (from Gibson). Specific value is the
value of the property divided by its density.
CHAPTER:3
Matrix and Filler Materials:
Polymers, metals and ceramics are all used as matrix materials in composites. The matrix
holds the fibers together in a structural unit,
protects them from external damage,
transfers and distributes the applied loads to the fibers, and
in many cases, contributes some needed property such as ductility, toughness, or
electrical insulation.
Because the matrix must transfer load to the fibers, a strong interface bond between the fiber and
matrix is extremely important; either through a mechanical or chemical bond between fibers and
matrix. Fibers and matrix must obviously be chemically compatible to prevent undesirable
reactions at the interface; this is especially important at high temperature where chemical
reactions can be accelerated.
Service temperature is quite often a controlling factor in consideration of a matrix material.
Listed in order of increasing temperature capability, we have:
Polymers are the most widely used matrix materials. They may be either thermosets (e.g.,
epoxy, polyester, phenolics) or thermoplastics (e.g., polyimide (PI), polyetheretherketone
(PEEK), polyphenylene sulfide (PPS)). Upon curing, thermosets form a highly cross-linked,
three-dimensional molecular network which does not melt at high temperature. Thermoplastics,
however, are based on polymer chains that do not cross-link. As a result, thermoplastics will
soften and melt at high temperature, then harden again upon cooling.
Epoxies and polyesters are also widely used. High grade epoxies are typically cured at about
350F and are generally not used at temperatures about 300F. The advanced thermoplastics
(PEEK, PI and PPS) have melting temperatures in the range of 600-700F. For higher
temperatures, metal, ceramic or carbon matrix materials are required.
Lightweight metals such as aluminum, titanium and magnesium and their alloys such titanium
aluminide and nickel aluminide may be used as matrix materials. For some of these, operating
temperature can be extended to about 2,250F. Advantages of metal matrices include higher
strength, stiffness and ductility (compared to polymers) but at the expense of higher density.
Ceramic matrix materials such as silicon carbide and silicon nitride can be use at temperatures
up to 3,000F. Hoever, ceramics have poor tensile strength are are quite brittle.
Carbon fiber/carbon matrix composites can be used at temperatures approaching 5,000F, but
the cost is such that they are only used in a few critical aerospace applications.
Filler materials are often used as a third component of a composite, and are typically mixed
with the matrix material during fabrication. Fillers do not typically enhance mechanical
properties but are used to alter or improve some other characteristic of the composite. Examples
include: hollow glass microspheres are used to reduce weight, clay or mica particles are used to
reduce cost, carbon black particles are used for protection against ultraviolet radiation, and
alumina trihydrate is used for flame and smoke suppression.
CHAPTER:4
The reasons why composites are selected for Aerospace Applications:
High strength to weight ratio (low density high tensile strength)
High creep resistance
High tensile strength at elevated temperatures
High toughness
Weight is everything when it comes to heavier-than-air machines, and designers have striven
continuously to improve lift to weight ratios since man first took to the air.Composite
materials have played a major part in weight reduction, and today there are three main types in
use: carbon fiber-, glass- and aramid- reinforced epoxy.; there are others, such as boron-
reinforced (itself a composite formed on a tungsten core).
Since 1987, the use of composites in aerospace has doubled every five years, and new
composites regularly appear.
Where Composite Are Used:
Composites are versatile, used for both structural applications and components, in all aircraft and
spacecraft, from hot air balloon gondolas and gliders, to passenger airliners, fighter planes and
the Space Shuttle. Applications range from complete airplanes such as the Beech Starship, to
wing assemblies, helicopter rotor blades, propellers, seats and instrument enclosures.
The types have different mechanical properties and are used in different areas of aircraft
construction. Carbon fiber for example, has unique fatigue behavior and is brittle, as Rolls Royce
discovered in the 1960's when the innovative RB211 jet engine with carbon fiber compressor
blades failed catastrophically due to birdstrikes.
Whereas an aluminium wing has a known metal fatigue lifetime, carbon fiber is much less
predictable (but dramatically improving everyday), but boron works well (such as in the wing of
the Advanced Tactical Fighter). Aramid fibers ('Kevlar' is a well-known proprietary brand owned
by DuPont) are widely used in honeycomb sheet form to construct very stiff, very light bulkhead,
fuel tanks and floors. They are also used in leading- and trailing-edge wing components.
In an experimental program, Boeing successfully used 1,500 composite parts to replace 11,000
metal components in a helicopter. The use of composite-based components in place of metal as
part of maintenance cycles is growing rapidly in commercial and leisure aviation.
Overall, carbon fiber is the most widely used composite fiber in aerospace applications.
The Future of Composites in Aerospace:
With ever increasing fuel costs and environmental lobbying, commercial flying is under
sustained pressure to improve performance, and weight reduction is a key factor in the equation.
Beyond the day-to-day operating costs, the aircraft maintenance programs can be simplified by
component count reduction and corrosion reduction. The competitive nature of the aircraft
construction business ensures that any opportunity to reduce operating costs is explored and
exploited wherever possible.
Competition exists in the military too, with continuous pressure to increase payload and range,
flight performance characteristics and 'survivability', not only of airplanes, but of missiles,
too.Composite technology continues to advance, and the advent of new types such as basalt and
carbon nanotube forms is certain to accelerate and extend composite usage.
Polymeric matrices in aerospace:
Thermoset resins
polyester , Epoxy,Phenol,Polyimide
Thermoplastics
PolyPhenyleneSulifide(PPS),
PolyEtherEtherKetone(PEEK),
PolyEtherimide (PEI)
Some composites in aerospace:
Metals (aluminium, titanium etc.)
Glass
Ceramics
Hybrid(composites)
Carbon aramid reinforced epoxy
Glass Carbon reinforced epoxy
Polymer-matrix composites are valued in the aerospace industry for their stiffness,
lightness, and heat resistance ( see materials science: Polymer-matrix composites).
They are fabricated materials in which carbon or hydrocarbon fibres (and sometimes
metallic strands, filaments, or particles) are bonded together by polymer resins in
either sheet or fibre-wound form.
The Boeing 777 is a long-range wide- body twin-engine jet airliner manufactured by Boeing
commercial airplanes. It is the world's largest twinjet and has a capacity of over 300
passengers, with a range of 5,235 to 9,380 nautical miles (9,695 to 17,370 km) .12%
composites are used.
Fig:4.3 &4.4
Showing
composite
material&ot-
her material
in various
parts on an
aeroplane.
CHAPTER:5
Advantage& Disadvantages of Composite :
We have already touched on a few, such as weight saving, but here is a full list:
Weight reduction - savings in the range 20%-50% are often quoted.
It is easy to assemble complex components using automated layup machinery and rotational
molding processes.
Monocoque ('single-shell') molded structures deliver higher strength at much lower weight.
Mechanical properties can be tailored by 'lay-up' design, with tapering thicknesses of
reinforcing cloth and cloth orientation.
Thermal stability of composites means they don't expand/contract excessively with change in
temperature (for example a 90°F runway to -67°F at 35,000 feet in a matter of minutes).
High impact resistance - Kevlar (aramid) armor shields planes, too - for example, reducing
accidental damage to the engine pylons which carry engine controls and fuel lines.
High damage tolerance improves accident survivability.
'Galvanic' - electrical - corrosion problems which would occur when two dissimilar metals are
in contact (particularly in humid marine environments) are avoided. (Here non-conductive
fiberglass plays a roll.)
Combination fatigue/corrosion problems are virtually eliminated.
CONCLUSION
Composite materials will play an increasingly significant role in aerospace application
B787,A350-XWB used more than 50% composite materials
With their unique combination of properties such as low weight, high strength, low
flammability, smoke density and heat release, non-toxicity and durability, composites are
ideal for many aerospace applications, both for interior and exterior components.